Message 23919 in TECHNI.WW
From: G8MNY
Date: Wed, 10 Jun 26 08:34:00 Z
Newsgroups: TECHNI.WW
Subject: HiFi Dynamic Noise Limiter
Message-ID: <62040_GB7CIP>
Path: N2MH4|VE3KPG|VE3CGR|PI8ZTM|CX2SA|OK2PEN|GB7CIP
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Clarification Mar 13)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is an old circuit I have used for years, only 4 transistors, originally
designed by Philips, as a hiss remover for tape recorders. I have used it for
the HiFi sound channel on 23cms ATV repeater GB3HV etc. Although treble gating
can sound bad if set up badly, with the right level of noise floor, it works
very well removing just the hiss & no useful treble.
Phase Inverting Treble Clipping Treble Treble
buffer Amp Amp Gate
Null
HOW IT WORKS
This is an unusual circuit. It uses treble phase cancellation to achieve the
high level of treble reduction (20dB) at low treble levels (-50dBm).
T1 produces 2 phases of signal, one via a fine level null control goes to the
output. The other feeds a treble Butterworth filter T2 then on to T3 that forms
a clipped treble amplifier. T4 buffers this and produces an amplified version
that drives 2 diodes D3 & 4 to quickly charge up C8 & 9 to control the treble
gate D5 & 6. This gate gently attenuates the out of phase treble so it cancels
it out if it is weaker than -40dBm.
SETUP
Set VR1 to provide the minimum hiss on a weak hiss source >-52dBm (2mV RMS).
Then you should have a noise reduction that is like this...
ATTENUATION LEVEL dBm mV
2k 3k 4k 5k 6k 7k 10k 15kHz FREQUENCY
So set up the input level to this circuit to give the noise floor @-52dBm (2mV)
so that you get maximum hiss attenuation. Then as programme material treble
content gets louder the noise gate gently reduces the top attenuation.
CONCLUSION
In mono it is very impressive for such a simple circuit. But for stereo some
combination of the left & right channel DC at C8 & C9 (e.g. a pair of 1k
bridges between the 2 channel's C8s & C9s) improves the stereo, by blurring the
the spatial image (stereo location) of the soft switching points.
Also see my Tech Bul "BHI Noise Eliminating DSP LS"
Why don't U sent an interesting Bul?
73 De John, G8MNY @ GB7CIP
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